Key error in relooper

```julia
using IRTools: @code_ir, CFG, reloop, explicitbranch!, @meta, IR

function test(i)
y = 0
for x in 1:i
y += x
end
return y
end

ir = explicitbranch!(IR(@meta(test(2)), slots = true));
reloop(ir)
```
Results in:
```julia
KeyError: key %4 not found
getindex at dict.jl:477 [inlined]
#203 at relooper.jl:109 [inlined]
prewalk(::IRTools.Inner.var"#203#204"{Dict{Any,Any}}, ::IRTools.Inner.Variable) at utils.jl:126
(::MacroTools.var"#21#22"{IRTools.Inner.var"#203#204"{Dict{Any,Any}}})(::IRTools.Inner.Variable) at utils.jl:126
iterate at generator.jl:47 [inlined]
collect_to!(::Array{GlobalRef,1}, ::Base.Generator{Array{Any,1},MacroTools.var"#21#22"{IRTools.Inner.var"#203#204"{Dict{Any,Any}}}}, ::Int64, ::Int64) at array.jl:667
collect_to_with_first!(::Array{GlobalRef,1}, ::GlobalRef, ::Base.Generator{Array{Any,1},MacroTools.var"#21#22"{IRTools.Inner.var"#203#204"{Dict{Any,Any}}}}, ::Int64) at array.jl:646
_collect(::Array{Any,1}, ::Base.Generator{Array{Any,1},MacroTools.var"#21#22"{IRTools.Inner.var"#203#204"{Dict{Any,Any}}}}, ::Base.EltypeUnknown, ::Base.HasShape{1}) at array.jl:640
map at array.jl:564 [inlined]
walk at utils.jl:105 [inlined]
prewalk at utils.jl:126 [inlined]
rename(::Dict{Any,Any}, ::Expr) at relooper.jl:108
ast(::IRTools.Inner.ASTCtx, ::IRTools.Inner.Block) at relooper.jl:136
ast(::IRTools.Inner.ASTCtx, ::IRTools.Inner.Simple) at relooper.jl:152
ast(::IRTools.Inner.ASTCtx, ::IRTools.Inner.Loop) at relooper.jl:212
(::IRTools.Inner.var"#nextblock#208"{IRTools.Inner.ASTCtx,IRTools.Inner.Simple,Dict{Any,Any}})(::IRTools.Inner.Branch) at relooper.jl:161
ast(::IRTools.Inner.ASTCtx, ::IRTools.Inner.Simple) at relooper.jl:177
#reloop#210(::Bool, ::typeof(IRTools.Inner.reloop), ::IRTools.Inner.IR) at relooper.jl:225
reloop(::IRTools.Inner.IR) at relooper.jl:222
top-level scope at untitled-c37c984d9c12c1d2362e9770c5fd862c:12
```